Field of the Invention
The present invention relates to a honeycomb structure that is used in filters for trapping particulate matters included in exhaust gases from diesel engines or gasoline engines, and the like, and especially relates to a honeycomb structure that can be preferably used in filters for which loading of a catalyst is required, and the like.
Description of the Related Art
Particulate matters (PM) are contained in exhaust gases from diesel engines and gasoline engines such as GDI (Gasoline Direct Injection) engines. The PM is mainly composed of carbon particulates such as soot and has been found to have a cancer-causing property. Therefore, it is necessary to prevent the PM from being released into the air, and thus a strict discharge regulation is imposed.
In order to correspond to such strict discharge regulation, various studies for decreasing the PM discharge amount have been conducted, but there is a limit to decrease the PM discharge amount by improving a combustion technology, and the only effective means for decreasing the PM discharge amount as of now is to install a filter in an exhaust system.
As a filter for trapping PM, wall flow type filters using a honeycomb structure are widely used since a high PM trapping efficiency can be obtained while a pressure loss is suppressed to be within an acceptable range. A honeycomb structure used in a wall flow type filter has a porous partition wall that defines a plurality of cells that extend from an inlet end face as an inlet side for an exhaust gas to an outlet end face as an outlet side for the exhaust gas, and a circumferential wall. By providing plugging portions that are configured to plug the open ends at the side of the outlet end face of the predetermined cells and the open ends of at the side of the inlet end face of the residual cells to this honeycomb structure, a filter having a high PM trap efficiency can be obtained.
Among such filters, gasoline particulate filters (GPFs), which are used for removing PM contained in exhaust gases from GDI engines, are used by loading a catalyst for purifying exhaust gases onto a partition wall in many cases. In such cases, honeycomb structures having a high porosity of 50% or more are used so that a pressure loss is suppressed to be within an acceptable range after the loading of the catalyst.
A relatively small honeycomb structure used for GPF is generally such that a partition wall and a circumferential wall are formed monolithically. Such honeycomb substrate is prepared by simultaneously forming the partition wall and the circumferential wall by extrusion molding, and firing the obtained formed article, and the partition wall and circumferential wall have an identical porosity.
In the case when a catalyst is loaded on a partition wall of a honeycomb structure, a slurry containing a catalyst (catalyst slurry) is introduced in cells by a conventionally-known aspiration process or the like to attach the slurry to the surface of the partition walls and pores, and a high temperature treatment is conducted to thereby fire the catalyst contained in the catalyst slurry on the partition walls. In the case when the honeycomb structure on which a catalyst is to be loaded has a high porosity as mentioned above and the circumferential wall and the partition wall have an identical porosity, the catalyst slurry that has been introduced into the cells may pass through the pores of the circumferential wall and exude on the outer surface of the circumferential wall. Furthermore, also in the case when a catalyst is loaded on a partition wall of a honeycomb structure in which the partition wall and a circumferential wall are separately formed, the catalyst slurry that has been introduced into the cells may exude on the outer surface of the circumferential wall if the circumferential wall has a porosity of 35% or more. In addition, there is a problem that, when such exudation of the catalyst slurry occurs, the workability deteriorates in the step of loading a catalyst on the partition wall of the honeycomb structure. Furthermore, the step of loading a catalyst on the honeycomb structure is conducted under a state in which a part of the circumferential wall of the honeycomb structure is chucked (gripped), but there is a problem that, if the circumferential wall has a high porosity, a sufficient strength cannot be obtained, and the circumferential wall is easily broken during the chucking. Furthermore, there is also a problem that, if the entirety of the honeycomb structure (partition wall and circumferential wall) has a high porosity, the isostatic strength of the honeycomb structure decreases, and thus the honeycomb structure is easily broken during transportation and actual use.
Conventionally, as a technology for improving the strength of the honeycomb structure, a technology for attaching a reinforcing material to a circumferential wall is known. For example, Patent Document 1 discloses a honeycomb structure in which the circumferential part of the honeycomb structure is reinforced with a material that disappears or scatters at a high temperature. Patent Document 2 discloses a honeycomb structure in which a material having approximately the same thermal expansion rate as that of a catalyst is attached to the entire outer surface of the circumferential wall of a ceramic honeycomb structure before loading a catalyst. Patent Document 3 discloses a honeycomb catalyst carrier in which an impregnated part is formed on the outermost periphery part at a predetermined thickness of an outermost circumference which is composed of a porous body and disposed so as to cover the circumferential part of a cell structural body. Here, the impregnated part is formed by impregnating the outermost periphery part with a non-water-soluble organic substance or inorganic substance that is lost by combustion.
[Patent Document 1] JP-A-2000-809
[Patent Document 2] JP-A-2001-871
[Patent Document 3] JP-A-2004-113887